Manufacture of thermionic devices



Aug. 8, I933. H, BEDFORD 1,921,067

MANUFACTURE OF THERMIONIC DEVICES Filed- Aug. 13, 1929 FIG.

FIG. .2

INVENTOR L H. ,BEDFORD WM 64 m6,

4 T TORNEV As'anexample of carrying outthe invention Patented Aug. 8, 1933 vim STATES 4 1,921,061 MANUFACTURE or rmannomc DEVICES Y Leslie Herbert Bedford, Aldwych, London, England, assignorto Western Electric Company,

Incorporated, New of New York York, N. Y., a Corporation Application Auguslls, 192e, Serial-No. 385,698

. and in Great Britain November 16, 1928 1 3 filaims. (or 250-275) In the manufacture of various articlesoicommerce 'such as thermionic devices, it is frequent-.'

ly necessary to produce an alkaline earth metal vapor inside the article at some stage of 'manufacture, usually during an evacuation process.

For this purpose two methods are well known.

in the art:

(1) A compound is provided which decomposes on heating to ape'rmanent gas and an alkaline earth metal. Such a compound is barium azide which is said to break down to nitrogen and metallic barium.

v of nitrogen-takes place when the temperature is raised to vaporize the barium, but also sets a limit to'the degree of vacuum which can be obtained in the finished article, on account of the vapor or dissociation pressure of the barium nitride which is deposited or formed in various parts of the device. In the second case the gastrouble theoretically absent, but in practice agreat' evolution of water vapor accompanies the reaction owing to the impomibility of keeping the barium oxide dry. There is also" a more serious objection that a-great deal of vaporization of magnesium takes place which inhibits the application of the method "in a great "mmanycases. V

The presentinventionprovides other sources of barium vapor whichare free or more nearly free from the above defects. V

According to'one feature the present invention providesa source of alkaline earth or rare earth metal vapors forl .thesaid' purpose-in, whichrthe metals are provided in a free con- 'dition, and provided whereinecessary with a suitable protection against oxidizing or hy- The metals are-preferably formed by the action of one or more reducing agents of removed by vacuo.

metallic barium may be prepared by a species of thermit reaction. A mixture of barium oxide,

barium peroxide, and metallic aluminum is heated, whereupon at a certain temperature (depending principally upon the proportions) a relic barium and-aluminum oxide. From this mixture it is comparatively easy to distil the barium in .a moderately good vacuum. The application to the thermionic device may take place at-any sta'gathat is to say either before or after the performance of the thermit reaction, or after the distillation of the barium. Measures may be provided to prevent oxidation or hydration of the source while it is being applied, for instance, to the plate of a valve. Saturated hydrocarbons carefully 'dried 'over sodium have been proved suitable as protection, but any other form of protection can be used, such'as another metal. Alternatively,- it may be preferable to extract the barium from the thermit product, barium and aluminum oxide, by dissolving the barium in another metal.

For example, the finely divided thermit product may be mixed with molten aluminum which takesup the barium in alloy form. A small piece of the resulting alloy serves as the barium source. A similar alloy may also be prepared electrolytically.-

In'the accompanying drawing, there is shown, in.Fig. 1, the envelope 1 of a thermionic or electric discharge device with an anode or plate member 2 and a filament or cathode member 3 therein, The source of thermionically active earth metal vapor in the form of a small pellet .4 may be introduced into the envelope 1 by; suitably fastening it to the anode. A source 5 of current external the envelope may be inserted in the cathode circuit by closing the switch 6 and theplate 2 heated by bombardment until to vaporize. Alternatively, the sourceof thermionically active earth metal may be heated by' action takes place with the formation of metalthe thermionically active earth metal is caused high frequency induction currents, the use of 1 which for such-purpose being wellknown. In-

stead of securing a pellet 4 to the plate as shown in Fig. 1, a recess 7 may be provided -in the anode 2 (see Fig. 2) in which the source-of theractive earth metal may be placed as a paste or in pellet form and covered by a cover plate 8 which permits the egress of the vaporized earth metal when the plate is heated to the necessary degree.

as already indicated while applying the por to the anode of the device, said source may be protected against hydration and oxidation by the use of a protective material, such as saturated hydrocarbons.

What is claimed is:

1. The method of producing a thermionically active earth metal vapor in an envelope of an electric discharge device which consists in introducing into the envelope an alumino-thermic reaction product containing pure thermionically active earth metal and aluminum oxide, and thereafter vaporizing therefrom, the pure thermionically active earth metal.

2. The method of producing a thermionically active earth metal vapor in an envelope of an source of thermionically active earth metal va- LESLIE HERBERT BEDFORD. 

